The present invention is directed to a system for automating corneal surgical procedures such as radial keratotomy, for example, and, more particularly, to means for accurately mapping the topography or surface configuration of the cornea and for automating and controlling the depth of cut made into the corneal tissue by the surgical knife.
Radial keratotomy, as well as other corneal surgical procedures, have experienced increased interest in an attempt to surgically correct refractive errors and thereby, if possible, remove the need for eyeglass correction. Radial keratotomy, originated by Fyodorov of the Soviet Union, is one of many new surgical techniques, along with keratomileusis, epikeratophakia and others, which have evolved in the past 25 to 30 years. Radial keratotomy achieves its results by flattening the cornea with radial incisions which begin paracentral to the corneal center, leaving an untouched central corneal zone of 3-4 mm in diameter, and extend peripherally. In the procedure currently practiced, 8 to 16 cuts are made into the thickness of the cornea. The primary function in creating these radial incisions on the external surface of the cornea is to reduce myopia (nearsightedness). At the present time it is estimated 100,000 patients have undergone the radial keratotomy operation in the United States.
While the complication rate is low and the operation relatively safe, the predictability of end results had been the major stumbling block. The operation is presently done on myopic patients ranging from 2 to 6 diopters of myopia. The results vary significantly from surgeon to surgeon although the same number of cuts from the edge of the same size optical zone may be done by each. A particularly important finding in many of the studies on the results of radial keratotomy has been the influence of the depth of incision on the degree of final refractive correction. Those procedures which had a relatively shallow cut produced the least amount of correction and the amount of ultimate regression of this effect was greatest in these patients. It has been learned that cutting approximately 90% of the thickness of the cornea throughout the entire length of the incision will produce the most flattening of the cornea and the most gain in the reduction of myopia. It should be here noted that the peripheral cornea is thicker than the paracentral and central corneal zones. This means that in order for a surgeon to cut the entire length of the incision at 90% thickness, he must redeepen the peripheral cut and he does this by random judgment. Redeepening procedure has the risk of perforation of the cornea and entrance into the anterior chamber of the eye. This significantly increases the risk for potential infections and complications such as damage to the corneal endothelial cell layer and the formation of cataracts.
Additional problems include the fact that the actual thickness of the cornea cannot be adequately measured by any known pachometer with consistent results. All instruments presently used vary in the reporting of the corneal thickness. It is estimated that the central cornea is approximately 500 microns and the peripheral cornea may be as thick as 580 or 600 microns. Since the current level of accuracy of optical and electronic pachometer measurements leave much to be desired in terms of accuracy, it is therefore virtually impossible for a surgeon to be able to consistently cut freehand at 90% of corneal thickness. (It should also be noted that corneal thicknesses vary from patient to patient.) Also thickness changes occur the moment the cornea is incised with the initial incision because the cornea incision will swell. At present all cutting of the cornea is done freehand by the surgeon in the following manner. The surgeon takes a pachometer reading and then, based on the site of the thinnest corneal reading, he sets the depth of cut of the knife that is to be used which may be either a diamond or a metal blade knife. Many surgeons presently set their blades at 100% corneal thickness in order to create the deepest cut. It is apparent that the exact depth of cut is not known in such circumstances and, since there is no penetration, it is equally apparent that either the pachometer readings are incorrect or the surgeon's setting of the knife blade is inaccurate. The desired result of obtaining a 90% thickness in cut over the full length of the incision cannot be achieved by this technique. Only a technique which takes into consideration the variation in corneal thickness in an unoperated upon cornea, as well as during the actual cutting procedure, can produce a 90% cut.
Another important area that is not fully understood, but which is important in understanding the result in terms of predictability, is the change in the topography (or surface contour) of the cornea. Measurement of corneal topography is required to define the corneal curvature before and after an operation. The anterior corneal refracting surface is the major focusing element on the eye. This surface has a power of about 49 diopters of convergence, and the posterior corneal surface has about 6 diopters of divergence. This corneal combination contributes about 43 diopters of convergence to the eye focusing systems, and the lens of the eye contributes about another 20 diopters of convergence. For a .+-.0.1 diopter accuracy, and a .+-.0.05 diopter repeatability requirement, a corneal topography measuring system will need to measure the distance between a reference and the corneal surface to an accuracy within 20 microns.
At the present time there is no satisfactory method to fully delineate during surgery, but prior to the actual cutting and postoperatively after the cutting, the true corneal topography. Photographic methods both conventional and photo electronic keratometers which essentially photograph light rings placed on the cornea, are inaccurate. Many of these do not reach the entire length of the cornea. They also cannot be used once the corneal surface has been disrupted and reflectivity has been altered, and thus they cannot be used during surgery. Measurements taken preoperatively by all of these conventional instruments obtain an average curvature prior to surgery and, therefore, fail to define clearly the true extent of changes in corneal topography in the peripheral cornea. The photoelectric keratometers only approximate the actual curvature changes on the corneal surface and do not truly define the exact relationship of the changes in curvature of the corneal surface. Therefore, at the present time there is no device that can be used intra-operatively which will accurately measure the complete corneal topography both prior to and immediately after incisions. In order to produce consistent results, a consistent cut and true topography measurement are necessary. It is only with the obtaining of this information that will there be an opportunity to correlate the final refractive change that has been created in a patient. Other factors such as age, scleral rigidity and intraocular tension, while playing some role, can be factored into the final predicted end result.